Heavy metal fluoride glass fibers are known to transmit wavelengths in the infrared region. Such fibers can have minimum optical losses in the range of 1-10 dB/km. Heavy metal fluoride fibers find applications in the fields of spectroscopy, laser power delivery in medicine and temperature sensing, and other fields using medium loss fibers (&lt;30 dB/km) which are optically satisfactory in short length applications. Major efforts have been made to minimize such optical loss and it is expected that heavy metal fluoride fibers may soon transmit telecommunication signals.
Unfortunately, heavy metal fluoride fibers have poor chemical durability, and are therefore not well suited for long term telecommunications applications. There thus exists a need for a heavy metal fluoride fiber which has low optical losses yet is chemically durable and provides a long useful lifetime.
Polyimide coatings are used to increase the chemical durability of silica fibers. Unfortunately, the polyimide coatings used in present silica fiber applications cure at temperatures which are far too high for fluoride fibers that have a glass transition temperature (Tg) of less than 300.degree. C. Typical curing temperatures (e.g. 400.degree.14 500.degree. C.) of current polyimide coatings are hundreds of degrees Celsius below the glass transition temperature of silica fibers. However, temperatures of 400.degree.14 500.degree. C. are sufficient to cause crystallization of heavy metal fluoride glass fibers which adversely affects the optical properties of the fiber.
The known polyimide solutions currently used include solutions of polyamic acid in 1-methyl-2-pyrrolidinone (NMP). Unfortunately, most known polyimide coatings cure at high temperatures. While some polyimide coatings have been made for temperature sensitive optical fibers, those coatings nonetheless cure at temperatures well above the glass transition temperature of most heavy metal fluoride fibers.
A need therefore exists for a protective coating which renders heavy metal fluoride fibers chemically durable. A need also exists for a method of applying a protective coating which cures without causing crystallization of the heavy metal fluoride fiber.